Diffusion pumps are characterized by a liquid pool of vaporizable diffusion pump fluid that is heated by a boiler to a vapor that rises through a jet assembly, and thence is emitted through at least one, and usually several, nozzles against a cool wall. The vapor is condensed on the cool wall to form a condensate that flows downwardly back to the periphery of the pool and thence inwardly toward the center of the pool due to convection currents in the pool. Most small diffusion pumps have heat applied to the pool from a source having a heating surface extending along the entire bottom of the pool. Usually, the heat source includes a flat electric heater that heats a metal plate having an upper surface which forms the bottom of the pool.
In the typical boiler, the vaporizing process tends to be uneven as convection currents in the pool are random, having horizontal and circular components that tend to be circular to cause local superheating of the fluid in the pool. In the areas of local superheating there is sudden and explosive boiling that changes the boiler pressure and the rate at which the liquid is vaporized and supplied to the jet assembly. The variations in the rate at which liquid is boiled from the pool are reflected in changes in speed of the pump and have a tendency to produce unstable pressure in a vacuum chamber being evacuated. If the vacuum load is a light gas, such as helium, the pressure instability may be particularly noticeable.
In one modified boiler, convection currents are relatively uniform and cause the liquid to flow from the periphery of the pool toward the center of the pool without local superheating of the liquid in the pool or pressure instability. This modified structure employs a heater having a cylindrical heating surface that is located in close proximity to the center of the pool and extends vertically through the pool. The heater is surrounded by a thin, annular shell having an open bottom so that cool liquid at the bottom of the pool enters an annular space between the shell and heater. The cool liquid is heated to a vapor as it rises through the annular space.
The basic problem with the modified boiler employing a central cylindrical heating surface is that it has a small heating surface, due to the relatively small radius of the cylindrical heater. To vaporize the liquid, the heat density (watts per square centimeter) on the surface of the cylinder must be very high, resulting in high temperatures in the annular region between the cylindrical heater and shell. The temperatures in the annular region are so high that there is a high probability of the fluid in the pool decomposing which adversely affects the ultimate pressure to which a chamber can be pumped.
A further disadvantage with the central, cylindrical heater is that the heater temperature becomes so high that an electrical heating element within the cylindrical heater has a tendency to become seized inside the cylinder. If the heating element is seized inside of the cylinder, removal of the heating element, when replacement is necessary, is very difficult and usually involves drilling out the heater element. The inner surface of the cylinder may be damaged by drilling, resulting in a poor contact between the new heater element and the cylinder wall.
It is, accordingly, an object of the present invention to provide a new and improved diffusion pump particularly adapted to pump light gases at a constant rate.
An additional object of the invention is to provide a new and improved diffusion pump wherein convection currents in a liquid being vaporized from a pool are relatively uniform and are not random.
An additional object of the invention is to provide a new and improved diffusion pump including a boiler wherein liquid is vaporized uniformly from a pool so that pressure of the vapor above the pool remains relatively constant.
An additional object of the invention is to provide a new and improved diffusion pump having a relatively constant speed and therefore relatively great pressure stability so that it is particularly adapted for use in connection with the pumping of light gases, such as helium.
Yet another object of the invention is to provide a new and improved diffusion pump employing a boiler having relatively uniform convection currents, and a heating surface with a relatively low watt density.
Still another object of the invention is to provide a new and improved diffusion pump wherein convection currents in a pool of liquid are relatively uniform and fluid in the pool does not have a tendency to decompose as a result of excessive temperatures being applied to it.
Yet a further object of the invention is to provide a new and improved diffusion pump having relatively uniform convection currents in a liquid pool and wherein a chamber can be pumped to a high vacuum, on the order of 6 .times. 10.sup.-9 torr.
Still another object of the invention is to provide a new and improved diffusion pump wherein relative uniform convection currents flow through a pool of liquid that is vaporized by a heat source which includes a heating element that is relatively easily replaced.